If the substrate used to produce integrated circuits already contains circuit elements, the relative alignment of mask and workpiece prior to the copying process must take the positions of these earlier-formed elements into account. The aforementioned reference markings may have, for example, the shape of transparent elongate windows whereas the associated adjustment marks may each comprise a linear groove in an SiO.sub.2 layer overlying the semiconductor substrate. The reference markings may include two pairs of such windows with major dimensions extending in mutually orthogonal directions. Upon proper alignment, light reflected from the workpiece images respective grooves in the windows of the mask.
Most visual-manual alignment methods employ polychromatic light as the alignment illumination. In such case, the light source is an incandescent lamp or a xenon lamp so that the spectrum of the alignment light ranges from about 500 nm into infrared. Alignment methods employing polychromatic light have, however, a number of disadvantages. In view of the fact that the image efficiency of projection lenses is greater when a beam of monochromatic light is used, most automatic alignment methods employ light having a relatively narrow spectrum (on the order of magnitude of some nm).
One of these prior-art systems uses for the alignment process the exposure light source, which is generally a mercury-vapor lamp. Either the narrow spectral range around 547 nm or that around 436 nm will then be used as the alignment light.
Still, a problem arises in this connection. If the alignment process is carried out by means of light at 436 nm, its intensity must be very low inasmuch as a photosensitive layer overlying the axial layer on the semiconductor disk is highly absorbent of and sensitive to wavelengths of less than 500 nm. That photosensitive layer should, however, not be pre-exposed by the alignment light.
This problem may be solved by using a separate alignment light source with a narrow spectrum or by changing to the higher spectral range around 547 nm. Apart from the costs of a separate alignment light source, the restriction to a precise alignment wavelength frequently creates a disadvantageous effect. Thus, because of interference phenomena, not only the actual mark but also its surroundings appear dark to the viewer so that the mark disappears or is visible only with inadequate contrast. The aforementioned interference phenomena are due to the fact that a semiconductor substrate employed for the production of integrated circuits is optically comparable to a reflective body of silicon coated with a partially reflective layer of silicon dioxide and a photoresist. In spite of the fact that the layer thicknesses are a multiple of the alignment wavelength, the light rays reflected by the first and second boundaries are extinguished by interference because of the great coherency length of the alignment light.